Crystallized glass substrate

ABSTRACT

To provide a crystallized glass substrate including a surface with a compressive stress layer, where a stress depth DOLzero of the compressive stress layer, at which the compressive stress is 0 MPa, is 45 to 200 μm, a compressive stress CS on an outermost surface of the compressive stress layer is 400 to 1400 MPa, and a central stress CT determined by using curve analysis is 55 to 300 MPa.

FIELD OF THE DISCLOSURE

The present disclosure relates to a crystallized glass substrateincluding a surface with a compressive stress layer.

BACKGROUND OF THE DISCLOSURE

A cover glass for protecting a display is used in a portable electronicdevice such as a smartphone or a tablet PC. A protector for protecting alens is also used in an in-vehicle optical device. In recent years,there is a demand for a use in a housing or the like serving as anexterior of an electronic device. There is an increasing demand for ahard and near-unbreakable material so that these devices can withstandmore rigorous use.

Conventionally, there is known chemical strengthening as a method forstrengthening a glass substrate. For example, Patent Document 1discloses a crystallized glass substrate for an information recordingmedium. However, when this crystallized glass substrate is chemicallystrengthened, it is not possible to achieve a sufficient compressivestress value.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Unexamined Patent Application PublicationNo. 2014-114200

SUMMARY OF THE DISCLOSURE

The present disclosure has been made in view of the above problem. Anobject of the present disclosure is to achieve a hard andnear-unbreakable crystallized glass substrate.

As a result of intensive studies to solve the above problem, the presentinventors discovered that it was possible to achieve a near-unbreakablecrystallized glass substrate having a high impact resistance, when asurface of the crystallized glass substrate included a predeterminedcompressive stress layer and had a predetermined central stress, and ledto completion of the present disclosure. Specifically, the presentdisclosure provides the following configurations.

(Configuration 1)

A crystallized glass substrate including a surface with a compressivestress layer, wherein

a stress depth DOL_(zero) of the compressive stress layer is 45 to 200μm, the stress depth DOL_(zero) being a depth at which the compressivestress is 0 MPa,

a compressive stress CS on an outermost surface of the compressivestress layer is 400 to 1400 MPa, and

a central stress CT determined by using curve analysis is 55 to 300 MPa.

(Configuration 2)

The crystallized glass substrate according to configuration 1, wherein asum of the stress depths from both surfaces of the crystallized glasssubstrate, 2×DOL_(zero), is 10 to 80% of a thickness T of thecrystallized glass substrate.

(Configuration 3)

The crystallized glass substrate according to the configuration 1 or 2,comprising: by wt % in terms of oxide,

40.0% to 70.0% of a SiO₂ component;

11.0% to 25.0% of an Al₂O₃ component;

5.0% to 19.0% of a Na₂O component;

0% to 9.0% of a K₂O component;

1.0% to 18.0% of one or more selected from a MgO component and a ZnOcomponent;

0% to 3.0% of a CaO component; and

0.5% to 12.0% of a TiO₂ component.

(Configuration 4)

The crystallized glass substrate according to any one of configurations1 to 3, wherein the thickness T of the crystallized glass substrate is0.1 to 1.0 mm.

(Configuration 5)

The crystallized glass substrate according to any one of configurations1 to 4, wherein E/ρ, which is a ratio of Young's modulus E (GPa) to aspecific gravity ρ is 31 or more.

(Configuration 6)

The crystallized glass substrate according to any one of configurations1 to 5, wherein a sum of the compressive stress CS on the outermostsurface and the central stress CT is 600 to 1400 MPa.

(Configuration 7)

The crystallized glass substrate according to any one of configurations1 to 6, wherein the stress depth DOL_(zero) is 70 to 110 μm,

the compressive stress CS of the outermost surface is 550 to 890 MPa,

the central stress CT is 100 to 250 MPa, and

the sum of the compressive stress CS on the outermost surface and thecentral stress CT is 800 to 1200 MPa.

(Configuration 8)

The crystallized glass substrate according to any one of configurations1 to 6, wherein the stress depth DOL_(zero) is 65 to 85 μm,

the compressive stress CS on the outermost surface is 700 to 860 MPa,

the central stress CT is 120 to 240 MPa, and

the thickness T of the crystallized glass substrate is 0.15 to 0.7 mm.

According to the present disclosure, it is possible to obtain a hard andnear-unbreakable crystallized glass substrate.

It is possible to use the crystallized glass substrate of the presentdisclosure for a display of an electronic device, a lens cover glass, anouter frame member or a housing, an optical lens material, and varioustypes of other members.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a frame used in a droptest in an example.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Embodiments and examples of a crystallized glass substrate of thepresent disclosure will be described below in detail, but the presentdisclosure is not limited to the following embodiments and examples, andmay be implemented with appropriate changes within the scope of theobject of the present disclosure.

[Crystallized Glass Substrate]

A crystallized glass substrate of the present disclosure usescrystallized glass as a base material (also referred to as “crystallizedglass base material”) and includes a surface with a compressive stresslayer. It is possible to form the compressive stress layer by subjectingthe crystallized glass base material to an ion exchange treatment. Thecompressive stress layer is formed from an outermost surface of thesubstrate toward the inside with a predetermined thickness, and acompressive stress is highest on the outermost surface, and decreasestoward the inside to near non-existence.

The compressive stress (also referred to as “outermost surfacecompressive stress”) CS of the outermost surface of the compressivestress layer is 400 to 1400 MPa, and may be 550 to 1300 MPa, 600 to 1200MPa, 650 to 1000 MPa, 700 to 890 MPa, 700 to 880 MPa, or 750 to 860 MPa,for example.

A depth DOL_(zero) (also referred to as “stress depth”) of thecompressive stress layer, at which the compressive stress is 0 MPa, is45 to 200 μm, and may be 50 to 140 μm, 55 to 120 μm, 65 to 110 μm, 70 to100 μm, or 75 to 85 μm, for example.

A sum of the stress depths from both surfaces of the crystallized glasssubstrate may be 10 to 80% of a thickness of the compressive stresslayer, and may be 12 to 60%, 15 to 50%, or 20 to 40%.

A central stress CT is 55 to 300 MPa, and may be 60 to 250 MPa, 65 to240 MPa, 80 to 230 MPa, 100 to 200 MPa, 105 to 180 MPa, or 120 to 150MPa, for example. It is noted that in the present disclosure, thecentral stress CT is determined by using curve analysis.

A sum of the outermost surface compressive stress CS and the centralstress CT may be 600 to 1400 MPa, 700 to 1200 MPa, 750 to 1100 MPa, or800 to 1000 MPa.

When the compressive stress layer has the above-mentioned stress depthDOL_(zero), the outermost surface compressive stress CS, and the centralstress CT, the substrate is substantially unbreakable. It is possible toadjust the stress depth DOL_(zero), the outermost surface compressivestress CS, and the central stress CT by adjusting a composition, asubstrate thickness, and a chemical strengthening condition.

A lower limit of a thickness of the crystallized glass substrate ispreferably 0.15 mm or more, more preferably 0.30 mm or more, still morepreferably 0.40 mm or more, yet still more preferably 0.50 mm or more,and an upper limit of the thickness of the crystallized glass substrateis preferably 1.00 mm or less, more preferably 0.90 mm or less, stillmore preferably 0.70 mm or less, and yet still more preferably 0.6 mm orless.

E/ρ, that is, a ratio of Young's modulus E (GPa) relative to a specificgravity ρ of the crystallized glass substrate, is preferably 31 or more,more preferably 32 or more, and still more preferably 33 or more.

The crystallized glass is a material having a crystal phase and a glassphase, and is distinguished from an amorphous solid. Generally, thecrystal phase of the crystallized glass is determined by using a peakangle appearing in an X-ray diffraction pattern of X-ray diffractionanalysis, and by using TEMEDX if necessary.

As the crystal phase, the crystallized glass contains, for example, oneor more selected from MgAl₂O₄, MgTi₂O₄, MgTi₂O₅, Mg₂TiO₄, Mg₂SiO₄,MgAl₂Si₂O₈, Mg₂Al₄Si₅O₁₈, Mg₂TiO₅, MgSiO₃, NaAlSiO₄, FeAl₂O₄, and solidsolutions thereof.

An average crystal diameter of the crystallized glass is, for example, 4to 15 nm, and may be 5 to 13 nm or 6 to 10 nm. If the average crystaldiameter is small, a surface roughness Ra after polishing may besmoothly processed up to about several Å levels. In addition, atransmittance increases.

A composition range of each component configuring the crystallized glassis described below. As used herein, a content of each component isexpressed in wt % in terms of oxide unless otherwise specified. Here,“in terms of oxide” means, if it is assumed that all the componentsincluded in the crystallized glass are dissolved and converted intooxides and a total weight of the oxides is 100% by weight, an amount ofoxides in each of the components contained in the crystallized glass isexpressed in wt %.

The crystallized glass serving as the base material preferably contains,in wt % in terms of oxide,

40.0% to 70.0% of a SiO₂ component;

11.0% to 25.0% of an Al₂O₃ component;

5.0% to 19.0% of a Na₂O component;

0% to 9.0% of a K₂O component;

1.0% to 18.0% of one or more selected from a MgO component and a ZnOcomponent,

0% to 3.0% of a CaO component; and

0.5% to 12.0% of a TiO₂ component.

The SiO₂ component is more preferably contained in an amount of 45.0% to65.0%, and still more preferably 50.0% to 60.0%.

The Al₂O₃ component is more preferably contained in an amount of 13.0%to 23.0%.

The Na₂O component is more preferably contained in an amount of 8.0% to16.0%. The Na₂O component may be contained in an amount of 9.0% or moreor 10.5% or more.

The K₂O component is more preferably contained in an amount of 0.1% to7.0%, and still more preferably 1.0% to 5.0%.

The one or more selected from the MgO component and the ZnO component ismore preferably contained in an amount of 2.0% to 15.0%, still morepreferably 3.0% to 13.0%, and particularly preferably 5.0% to 11.0%. Theone or more selected from the MgO component and the ZnO component may bethe MgO component alone, the ZnO component alone, or both of thecomponents, but preferably the MgO component alone.

The CaO component is more preferably contained in an amount of 0.01% to3.0%, and still more preferably 0.1% to 2.0%.

The TiO₂ component is more preferably contained in an amount of 1.0% to10.0%, and still more preferably 2.0% to 8.0%.

The crystallized glass may contain 0.01% to 3.0% (preferably 0.1% to2.0%, more preferably 0.1% to 1.0%) of one or more selected from theSb₂O₃ component, the SnO₂ component, and the CeO₂ component.

The above blending amounts may be combined as appropriate.

The crystallized glass may contain one or more selected from the SiO₂component, the Al₂O₃ component, the Na₂O component, the MgO component,and the ZnO component; and the TiO₂ component in an amount of 90% ormore, preferably 95% or more, more preferably 98% or more, and stillmore preferably 98.5% or more.

The crystallized glass may contain one or more selected from the SiO₂component, the Al₂O₃ component, the Na₂O component, the K₂O component,the MgO component, and the ZnO component; the CaO component; the TiO₂component; and one or more selected from the Sb₂O₃ component, the SnO₂component, the CeO₂ component in an amount of 90%, preferably 95% ormore, more preferably 98% or more, and still more preferably 99% ormore. The crystallized glass may consist only of these components.

The crystallized glass may or may not contain a ZrO₂ component as longas the effect of the present disclosure is not impaired. The blendingamount may be 0 to 5.0%, 0 to 3.0%, or 0 to 2.0%.

As long as the effect of the present disclosure is not impaired, thecrystallized glass may or may not contain a B₂O₃ component, a P₂O₅component, a BaO component, a FeO component, a SnO₂ component, a Li₂Ocomponent, a SrO component, a La₂O₃ component, a Y₂O₃ component, a Nb₂O₅component, a Ta₂O₅ component, a WO₃ component, a TeO₂ component, and aBi₂O₃ component. The blending amount of each of the components may be 0to 2.0%, 0 or more and less than 2.0%, or 0 to 1.0%.

The crystallized glass of the present disclosure may or may not contain,as a clarifying agent, a Sb₂O₃ component, a SnO₂ component, a CeO₂component, and an As₂O₃ component, and in addition, one or two or morekinds selected from the group consisting of F, Cl, NOx, and SOx.However, a content of the clarifying agent is preferably 5.0% or less,more preferably 2.0% or less, and most preferably 1.0% or less.

The crystallized glass serving as the base material preferably contains,in mol % in terms of oxide,

43.0 mol % to 73.0 mol % of a SiO₂ component;

4.0 mol % to 18.0 mol % of an Al₂O₃ component;

5.0 mol % to 19.0 mol % of a Na₂O component;

0 mol % to 9.0 mol % of a K₂O component;

2.0 mol % to 22.0 mol % of one or more selected from a MgO component anda ZnO component,

0 mol % to 3.0 mol % of a CaO component; and

0.5 mol % to 11.0 mol % of a TiO₂ component.

The crystallized glass may contain one or more selected from the SiO₂component, the Al₂O₃ component, the Na₂O component, the MgO component,and the ZnO component; and the TiO₂ component in an amount of 90 mol %or more, preferably 95 mol % or more, more preferably 98 mol % or more,and still more preferably 99 mol % or more.

Other components not described above may be added to the crystallizedglass of the present disclosure, if necessary, as long as thecharacteristics of the crystallized glass of the present disclosure arenot impaired. For example, the crystallized glass (and the substrate) ofthe present disclosure may be colorless and transparent, but the glassmay be colored as long as the characteristics of the crystallized glassare not impaired.

There is a tendency that use of each component of Pb, Th, Tl, Os, Be andSe, which is considered, in recent years, as a harmful chemicalsubstance, and is prevented, and therefore, it is preferable that thecomponent is not substantially contained.

In a drop test conducted in Examples, the crystallized glass substrateof the present disclosure preferably has a breakage height of 60 cm ormore, 70 cm or more, 80 cm or more, 90 cm or more, 100 cm or more, or110 cm or more.

[Producing Method]

It is possible to produce the crystallized glass substrate of thepresent disclosure by the following method. That is, a raw material isuniformly mixed and the mixed raw material is melting and forming toproduce raw glass. Next, the resultant raw glass is crystallized toproduce a crystallized glass base material. Further, the crystallizedglass base material is chemically strengthened.

The raw glass is heat-treated to precipitate crystals into the glass.The raw glass may be heat-treated at a one-step temperature or atwo-step temperature.

In a two-step heat treatment, a nucleation step is firstly performed byheat treatment at a first temperature, and after the nucleation step, acrystal growth step is carried out by heat treatment at a secondtemperature higher than that in the nucleation step.

In a one-step heat treatment, the nucleation step and the crystal growthstep are continuously performed at the one-step temperature. Typically,the temperature is raised to a predetermined heat treatment temperature,the temperature is maintained for a certain period of time afterreaching the heat treatment temperature, and the temperature is thenlowered.

A first temperature of the two-step heat treatment is preferably 600° C.to 750° C. A holding time at the first temperature is preferably 30minutes to 2000 minutes, and more preferably 180 minutes to 1440minutes.

A second temperature of the two-step heat treatment is preferably 650°C. to 850° C. A holding time at the second temperature is preferably 30minutes to 600 minutes, and more preferably 60 minutes to 300 minutes.

When the heat treatment is performed at the one-step temperature, theheat treatment temperature is preferably 600° C. to 800° C., and morepreferably 630° C. to 770° C. A holding time at the heat treatmenttemperature is preferably 30 minutes to 500 minutes, and more preferably60 minutes to 300 minutes.

From the crystallized glass base material, it is possible to produce athin plate-shaped crystallized glass base material by using, forexample, grinding and polishing means.

After that, a compressive stress layer is formed on the crystallizedglass base material through ion exchange by a chemical strengtheningmethod.

It is possible to obtain the crystallized glass substrate of the presentdisclosure by chemically strengthening the crystallized glass basematerial at a predetermined temperature and for a predetermined time ina molten potassium salt (single bath) (including one or more kinds ofpotassium salts, such as potassium nitrate (KNO₃), potassium carbonate(K₂CO₃), and potassium sulfate (K₂SO₄)) rather than a mixed molten salt(mixed bath) including potassium salt and sodium salt. For example, thecrystallized glass base material is contacted or immersed in a moltensalt heated to 450 to 580° C. (500 to 550° C. or 520 to 530° C.), for380 minutes to 630 minutes, 400 minutes to 600 minutes, 450 to 550minutes, or 480 to 520 minutes, for example. By such chemicalstrengthening, an ion exchange reaction between a component present nearthe surface and a component contained in the molten salt proceeds, andas a result, the compressive stress layer having the abovecharacteristics is formed on the surface. In particular, when thecrystallized glass base material is strengthened at 500 to 550° C. for480 to 520 minutes, it is more likely to obtain a near-unbreakablesubstrate.

EXAMPLES Examples 1 to 11 and Comparative Example 1

In Examples 1 to 11, raw materials such as oxides, hydroxides,carbonates, nitrates, fluorides, chlorides, and metaphosphate compoundseach of which corresponds to a raw material of each component of thecrystallized glass are selected, and these raw materials are weighed andmixed uniformly to have the following composition ratios.

(wt % in Terms of Oxide)

A SiO₂ component is 54%, an Al₂O₃ component is 18%, a Na₂O component is12%, a K₂O component is 2%, a MgO component is 8%, a CaO component is1%, and a TiO₂ component is 5%, and an Sb₂O₃ component is 0.1%

Next, the mixed raw materials were fed and melted into a platinumcrucible. Thereafter, the molten glass was stirred and homogenized, castinto a mold, and slowly cooled to produce raw glass.

The obtained raw glass was subjected to a one-step heat treatment (at650 to 730° C., for five hours) for nucleation and crystallization toproduce crystallized glass serving as a base material. As a result ofanalyzing the obtained crystallized glass with a 200 kV field emissiontransmission electron microscope FE-TEM (JEM 2100F manufactured by JEOLLtd.), precipitated crystals having an average crystal diameter of 6 to9 nm were observed. Further, a lattice image was confirmed through anelectron diffraction image and the obtained crystallized glass wasanalyzed by EDX, and crystal phases of MgAl₂O₄ and MgTi₂O₄ wereconfirmed. Crystal diameters of crystal particles in a range of 180×180nm² are determined by using a transmission electron microscope tocalculate an average crystal diameter.

The produced crystallized glass base material was cut and ground to havea shape of 150 mm in length, 70 mm in width, and 1.0 mm or more inthickness, and the opposing sides of the produced crystallized glassbase material were polished so as to be parallel to each other. Thecrystallized glass base material was colorless and transparent.

The opposing sides of the produced crystallized glass base materialpolished so as to be parallel to each other to obtain the thicknessshown in Table 1 was chemically strengthened to obtain a crystallizedglass substrate including a surface with a compressive stress layer.Specifically, the crystallized glass base material was immersed in aKNO₃ molten salt at a salt bath temperature and in an immersion timeshown in Table 1.

In Comparative Example 1, a general chemically strengthened glasssubstrate having the following composition was used. It is consideredthat this substrate was immersed in a mixed bath including KNO₃ andNaNO₃ and then immersed in a single bath including KNO₃.

(wt % in Terms of Oxide)

A SiO₂ component is 54%, an Al₂O₃ component is 13%, a Na₂O component is5%, a K₂O component is 17%, a MgO component is 5.5%, a CaO component is0.5%, and a B₂O₃ component is 3%, and a ZrO₂ component is 2%

A compressive stress value (CS) (MPa) and a stress depth (DOL_(zero))(μm) on an outermost surface of the compressive stress layer of thecrystallized glass substrate were measured by using a glass surfacestress meter FSM-6000LE manufactured by Orihara Manufacturing Co., LTD.A refractive index of 1.54 and an optical elastic constant of 29.658[(nm/cm)/MPa] were used to calculate the compressive stress value (CS)(MPa) and the stress depth (DOL_(zero)) (μm). A central stress value(CT) (MPa) was determined by using curve analysis. Table 1 also shows asubstrate thickness (T) (mm), a ratio of DOL_(zero) (a sum of DOL_(zero)values from both surfaces of the substrate) to a substrate thickness (T)(2DOL_(zero)/1000 T×100), and a sum of an outermost surface compressivestress value and the central stress value (CS+CT) (MPa).

A steel ball drop test was performed on the crystallized glass substrateby the following method.

An acrylic frame 1 was used, and FIG. 1 illustrates a section of theframe 1. The frame 1 includes a rectangular outer frame 10 and an innerframe 20 lower than the outer frame 10, the outer frame 10 and the innerframe 20 forms a step, and the interior of the inner frame 20 is vacant.An inner size of the outer frame 10 is 151 mm×71 mm, and an inner sizeof the inner frame 20 is 141 mm×61 mm. The crystallized glass substrate30 was placed inside the outer frame 10 and on the inner frame 20. A 130g stainless steel ball was dropped from a height of 10 cm from thecrystallized glass substrate 30. If the substrate 30 did not break afterthe drop, the height was increased by 10 cm and the test was continuedin much the same way until the substrate 30 was broken. Breakage heightsare shown in Table 1. From Table 1, it can be seen that the substrate 30in Examples are near unbreakable.

Further, Young's modulus E (GPa) and a specific gravity ρ were measured,and a ratio E/ρ therebetween was determined. The Young's modulus wasmeasured by an ultrasonic method. The results are shown in Table 1.

TABLE 1 Comparative Example 1 2 3 4 5 6 7 8 9 10 11 Example 1 Salt bath520 530 550 520 530 550 460 500 530 540 550 — temperature (° C.)Immersion 500 500 500 500 500 500 500 500 500 500 500 time (minutes)Substrate 0.5 0.5 0.5 0.7 0.7 0.7 0.8 0.8 0.8 0.8 0.8 0.65 thickness T(mm) CS (MPa) 813 754 598 852 785 647 1118 973 797 733 652 742 DOLzero74 79 92 81 88 106 47 70 90 98 107 63 (μm) DOLzero 30 31 37 23 25 30 1218 23 24 27 19 ratio (%) CT (MPa) 181 190 225 125 135 162 63 94 116 123134 53 CS + CT 994 944 822 977 920 810 1180 1067 913 856 786 795 (MPa)Drop test 150 120 100 110 70 90 100 100 90 100 60 50 height (cm) Young's86 73 modulus E (Gpa) Specific 2.54 2.46 gravity ρ E/ρ 33.9 29.7

Although some embodiments and/or examples of the present disclosure aredescribed above in detail, those skilled in the art may easily applymany modifications to these exemplary embodiments and/or exampleswithout substantial departure from the novel teachings and effects ofthe present disclosure. Therefore, many of these modifications arewithin the scope of the invention.

All the contents of the literature described in the specification areincorporated herein.

What is claimed is:
 1. A crystallized glass substrate including asurface with a compressive stress layer, wherein a stress depthDOL_(zero) of the compressive stress layer is 45 to 200 μm, the stressdepth DOL_(zero) being a depth at which a compressive stress is 0 MPa, acompressive stress CS on an outermost surface of the compressive stresslayer is 400 to 1400 MPa, and a central stress CT determined by usingcurve analysis is 55 to 300 MPa; wherein by wt % in terms of oxide, thecrystallized glass substrate includes: 40.0% to 70.0% of a SiO₂component; 11.0% to 25.0% of an Al₂O₃ component; 5.0% to 19.0% of a Na₂Ocomponent; 0% to 9.0% of a K₂O component; 1.0% to 18.0% of one or moreselected from a MgO component and a ZnO component; 0% to 3.0% of a CaOcomponent; 0.5% to 12.0% of a TiO₂ component, and 0% to 2.0% of a Li₂Ocomponent; wherein the crystallized glass substrate includes acrystallized glass having a crystal phase containing one or more ofMgAl₂O₄, MgTi₂O₄, MgTi₂O₅, Mg₂TiO₄, Mg₂SiO₄, MgAl₂Si₂O₈, Mg₂Al₄Si₅O₁₈,Mg₂TiO₅, MgSiO₃, NaAlSiO₄, FeAl₂O₄, and solid solutions thereof; whereinan inner size of the outer frame is 151 mm×71 mm, and an inner size ofthe inner frame is 141 mm×61 mm, a steel ball drop test is constitutedby dropping a 130 g stainless steel ball on the crystallized glasssubstrate placed inside the outer frame and on the inner frame, and thecrystallized glass substrate has a breakage height of 60 cm or more. 2.The crystallized glass substrate according to claim 1, wherein a sum ofthe stress depths from both surfaces of the crystallized glasssubstrate, 2×DOL_(zero), is 10 to 80% of a thickness T of thecrystallized glass substrate.
 3. The crystallized glass substrateaccording to claim 1, wherein a thickness T of the crystallized glasssubstrate is 0.1 to 1.0 mm.
 4. The crystallized glass substrateaccording to claim 1, wherein E/ρ, which is a ratio of Young's modulus E(GPa) to a specific gravity ρ, is 31 or more.
 5. The crystallized glasssubstrate according to claim 1, wherein the compressive stress CS of theoutermost surface is 550 to 1400 MPa.
 6. The crystallized glasssubstrate according to claim 1, wherein the stress depth DOL_(zero) is70 to 110 μm, the compressive stress CS of the outermost surface is 550to 890 MPa, the central stress CT is 100 to 250 MPa, and a sum of thecompressive stress CS on the outermost surface and the central stress CTis 800 to 1200 MPa.
 7. The crystallized glass substrate according toclaim 1, wherein the stress depth DOL_(zero) is 65 to 85 μm, thecompressive stress CS on the outermost surface is 700 to 860 MPa, thecentral stress CT is 120 to 240 MPa, and a thickness T of thecrystallized glass substrate is 0.15 to 0.7 mm.